In this chapter, we will start to develop a simple game. The game is the Mastermind game for two players. Player one selects four differently colored pins out of six possible colors and arranges them on a board in a row hidden from the other player. The other player tries to guess the colors of the pins and its positions. After each try, player one tells the number of matching colors and the pins matching both color and position. The program will act as both player one and player two. Our code will play alone. However, what remains for us to play with is the most important: the code.

This example is complex enough to deepen the OO principles and how we design classes and model the real world. We have already used classes provided in the Java runtime. This time, we will use collections and discuss this important area. These classes and interfaces are widely used and available in the JDK and as important for a professional Java developer as the language itself.

The build tool this time is Gradle.

In this chapter we will cover:

• Java collections
• Dependency injection
• How to comment our code and to create JavaDoc documentation
• How to create integration tests